1. Field of the Invention
The present invention relates to a pattern detector used in precise position measurement of a wafer or a mask in a mask aligner during the manufacture of a semiconductor device.
2. Description of the Prior Art
FIG. 1 shows a pattern detector in a prior art reduction projection aligner system. A similar device is disclosed in Japanese Patent Application Laid-Open No. 162227/80 (corresponding to U.S. Pat. No. 4,380,395).
In the reduction projection aligner system of FIG. 1, a circuit pattern of a reticle 2 to be newly formed is superimposed on a circuit pattern on a wafer 4 formed in a previous step through an exposing condenser lens 1 and a reduction lens 3. Usually, the superimposing step is repeated for a certain number of reticles to form a desired circuit pattern on the wafer. Usually, a high precision of no less than 1 .mu.m is required for the registration of the two circuit patterns to be superimposed. In the reduction projection aligner system shown in FIG. 1, the registration is effected by detecting a positioning pattern on the wafer and relatively moving the reticle having the pattern to be newly formed so that it coincides with the wafer. An area containing the positioning pattern (not shown) on the wafer 4 is locally illuminated by a light ray emanated from a positioning pattern detection light source (not shown) and guided by a light guide 10, and a light reflected thereby is magnified and focused through the reduction lens 3, the reticle 2, a mirror 100 and a magnifying optical system 5 on a moving plane of a reciprocating table 7 having a slit 6. The reciprocating table 7 is reciprocated in the direction of arrow shown in the figure and the magnified image of the area containing the positioning pattern on the wafer 4 is scanned by the slit 6 reciprocating in accordance with the movement of the reciprocating table 7. The intensity of illumination of the magnified image is applied to a photo-multiplier 9 as a transmitted light from the slit and it is photo-electrically converted. An output of the photo-multiplier 9 is digitized by a circuit including an A-D converter, a digital counter and a CPU and an illumination-intensity data of points corresponding to slit positions of the magnified image is produced. Numeral 8 denotes a portion of a mechanism for generating an input signal to the digital counter. The wafer position (that is, a center position of the positioning pattern on the wafer 4) can be determined in the following manner (see Japanese Patent Application Laid-Open No. 69063/78, corresponding to U.S. Pat. No. 4,115,762). An arbitrary position Xi of the slit is assumed as a virtual center and 2m intensity data Yi on both sides thereof are superimposed to calculate Zi= ##EQU1## A point which provides a minimum one of the values of Z is determined as the center position of the positioning pattern on the wafer. In this device, the center position X.sub.c of the positioning pattern on the wafer relative to an origin sensor (not shown) located at a position on the reciprocating table 7 corresponding to a center position of the reticle reference pattern is determined and the new pattern is superimposed on the pattern on the wafer in accordance with the result of the determination.
In the above reduction projection aligner system, when the pattern position is detected through the reduction lens 3, a detecting light wavelength is an exposure wavelength of monochromatic light because the reduction lens 3 is compensated for aberration for the exposure wavelength. When a monochromatic light is applied to a wafer having an optically transparent thin film photoresist applied thereon, the light reflected by the surface of the wafer is repeatedly reflected between the wafer surface and the photoresist surface and goes out of the photoresist surface. An interference occurs among the light from the photoresist surface, the light from the wafer surface and the repeatedly reflected light in the photoresist, and a so-called interference fringe of equal thickness occurs. Accordingly, when the monochromatic light such as exposure light is used as the pattern detecting light, the interference fringe of equal thickness occurs depending on the thickness of the photoresist. This results in a reduction of a contrast of the detection signal by dark areas of the interference fringe and hence the precision of the pattern positioning is lowered. Further, since the reflection of the exposure light by the wafer impedes the formation of a fine pattern, an anti-reflection film is overlayed on the wafer or light absorbing agent for the exposure light wavelength is added in the photoresist. In such a case, a sufficient intensity of reflected light from the wafer is not obtained if the detection is made using the exposure light wavelength. In order to resolve this problem, it is necessary to detect the pattern by a light of different wavelength than the exposure light wavelength. To this end, a color aberration correction lens is disposed between the reduction lens 3 and the reticle 2 or a special pattern detecting optical system adapted exclusively to a color aberration of the reduction lens 3 is prepared to detect the pattern only by a light of a specific wavelength. As a result, a pattern detection error occurs due to a reproduction error of a correction lens position. Even if the light of different wavelength than the exposure light wavelength is used, the pattern detecting magnifying optical system for a single wavelength cannot vary the detecting wavelength depending on the thickness of the photoresist on the wafer, the material of the photoresist and the material of the wafer to detect the pattern at an optimum wavelength. Namely, it cannot detect the pattern at any desired wavelength.